Past tenses

I have always assumed that the past tenses follow an order in time from earliest to latest as follows: pluperfect, perfect, imperfect. I order the last two in that way because the action is completed in perfect and not in imperfect.

I recently saw this:

Before the general entered the town, we feared him.

translated as:
ducem, antequam oppidum intravit, timebamus.

The use of the imperfect here has confused me as I did not expect to see it before a perfect event.

The imperfect doesn't really have a fixed point in that order, since it conveys a different aspect than the other two. Here, we have something that literally means "We used to fear the general before he entered the town". This "used to" is where the imperfect comes in.

I have always assumed that the past tenses follow an order in time from earliest to latest as follows: pluperfect, perfect, imperfect. I order the last two in that way because the action is completed in perfect and not in imperfect.

That is a total misunderstanding of the tenses, which you have invented in your own little head, lol

No textbook taught you anything like that. The notion is so far off from the true distinction that it will make you completely misuse the tenses as long as you labour under it.

The use of tenses in that extract is exactly what one would expect.

Go back to the first chapter of your textbook that introduces the perfect-imperfect contrast, and work from there.